Electro-optical device

The present invention relates to an electro-optical device used in the fields of optical communication and optical instrumentation.

Communication traffic has been remarkably increased with widespread Internet use, and optical fiber communication is becoming significantly important. The optical fiber communication is a technology that converts an electric signal into an optical signal and transmits the optical signal through an optical fiber and has wide bandwidth, low loss, and resistance to noise.

Optical fiber communication has such advantages and is applied to various products, and its representative can be, for example, optical switching devices or optical modulators. In particular, as a method for converting an electric signal into an optical signal, there are known a direct modulation system using a semiconductor laser and an external modulation method using an optical modulator. The direct modulation does not require the optical modulator and is thus low in cost, but has a limitation in terms of high-speed modulation and, thus, the external modulation method is used for high-speed and long-distance applications.

As the optical modulator, a Mach-Zehnder optical modulator in which an optical waveguide is formed by Ti (titanium) diffusion in the vicinity of the surface of a single-crystal lithium niobate substrate has been practically used (see, e.g., Patent Document 1). Although high-speed optical modulators having a modulation speed of 40 Gb/s or more are commercially available, they have a major drawback that the entire length thereof is as long as about 10 cm. The Mach-Zehnder optical modulator is an optical modulator that uses an optical waveguide (Mach-Zehnder optical waveguide) having a Mach-Zehnder interferometer structure. The Mach-Zehnder interferometer is a device that separates light emitted from one light source into two beams, makes the two beams pass through different paths, and then recombines the two beams to cause interference, and the Mach-Zehnder optical modulator applying the Mach-Zehnder interferometer is used for generating various modulated lights.

5 FIG. 300 22 22 21 24 24 22 22 23 300 24 24 24 22 22 22 22 a b a b a b a a b a b a b In contrast, Patent Document 2 discloses a Mach-Zehnder optical modulator using a lithium niobate film. The optical modulator using the lithium niobate film achieves significant reduction in size and driving voltage as compared with an optical modulator using the lithium niobate single-crystal substrate.shows a cross-sectional structure of a conventional optical modulatordescribed in Patent Document 2. A pair of optical waveguidesandof a lithium niobate film are formed on a sapphire substrate, and a signal electrodeand a ground electrodeare disposed above the optical waveguidesand, respectively, through a buffer layer. The optical modulatoris a so-called single drive type having one signal electrode, and the signal electrodeand ground electrodehave a sym-metrical structure, so that electric fields to be applied to the optical waveguidesandare the same in magnitude and opposite in polarity. However, the cross-sectional shape of the optical waveguides,is rectangular, and the propagation loss of light is large.

Patent Document 3 discloses a portion where the optical waveguide needs to be bent in the case of the Mach-Zehnder optical modulator. In order to prevent the loss of the bent portion, it is necessary to further enhance the locked-in of light, and a technique of forming a ridge-shaped optical waveguide has been disclosed.

In order to reduce the light propagation loss by reducing the size of the optical modulator, it is necessary to reduce the absorption and reflection of light leaking from the optical waveguide in the ridge-shaped LN film optical waveguide and to effectively apply an electric field from the electrode on the layer to the optical waveguide.

Patent Document 1: Japanese Patent No. 4485218 Patent Document 2: JP 2006-195383A Patent Document 3: JP 2007-328257A

The present invention has been completed in view of the above-mentioned problems, and its object is to provide a electro-optical device, comprising: a substrate; an optical waveguide composed of an electro-optic material film formed in a ridge shape on the substrate; a buffer layer configured to cover the optical waveguide; and an upper electrode provided on the optical waveguide through the buffer layer, and the buffer layer has a recess on the upper electrode side above the optical waveguide.

In addition, another object of the present invention is to provide an electro-optical device with a small light propagation loss, comprising: a substrate; first and second optical waveguides adjacent to each other, composed of an electro-optic material film formed in a ridge shape on the substrate; a buffer layer configured to cover the first and second optical waveguides and to be buried between the first and second optical waveguides; and first and second electrodes arranged to be opposite to the first and second optical waveguides above the buffer layer, the buffer layer has recesses above the first and second optical waveguides.

In addition, another object of the present invention is to provide an electro-optical device, comprising: a substrate; first and second optical waveguides adjacent to each other, composed of an electro-optic material film formed in a ridge shape on the substrate; a buffer layer configured to cover the first and second optical waveguides and to be buried between the first and second optical waveguides; and first and second electrodes arranged to be opposite to the first and second optical waveguides above the buffer layer, the distance from the surface of the substrate to the uppermost part of the buffer layer on the optical waveguide is smaller than the distance from the surface of the substrate to the uppermost part of the buffer layer on the portion where the optical waveguide is not formed.

In addition, in the electro-optical device of the present invention, it is preferable that the buffer layer provided between the first and second optical waveguides and the first and second electrodes has a shape protruding downward at the first and second optical waveguides.

In addition, in the electro-optical device of the present invention, it is preferable that the buffer layer provided between the first and second optical waveguides and the first and second electrodes has a shape bent downward at the first and second optical waveguides.

In addition, in the electro-optical device of the present invention, it is preferable that the amount of recess of the buffer layer on the first and second optical waveguides is 500 Angstrom or more, more preferably 1000 Angstrom to 10000 Angstrom.

In addition, in the electro-optical device of the present invention, it is preferable that the first and second optical waveguides are Mach-Zehnder optical waveguides.

In addition, in the electro-optical device of the present invention, it is preferable that the substrate is a single crystal substrate, the electro-optic material film is a lithium niobate film.

According to the electro-optical device of the present invention, an electric field can be effectively applied to the optical waveguide, thereby locking light in the optical waveguide and suppressing the propagation loss of the light.

Preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

1 1 a b FIGS.() and() 1 a FIG.() 1 b FIG.() 100 100 are plan views of an optical modulator (electro-optical device)according to the first embodiment of the present invention,illustrates only the optical waveguide, andshows the entire of the optical modulatorincluding traveling wave electrodes.

1 a FIG.() 1 b FIG.() 100 10 1 10 10 7 10 8 10 a b a b. As illustrated inand, the optical modulatorincludes a Mach-Zehnder optical waveguideformed on a substrateand having first and second optical waveguides,provided in parallel to each other; a first electrodeprovided along the first optical waveguide; and a second electrodeprovided along the second optical waveguide

10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 a b i c a b o d c a b d The Mach-Zehnder optical waveguideis, for example, an optical waveguide having a structure of a Mach-Zehnder interferometer. The Mach-Zehnder optical waveguidehas the first and second optical waveguides,which are branched from a single input optical waveguideat a demultiplexing section, and the first and second optical waveguides,are combined into a single output optical waveguideat a multiplexing section. An input light Si is demultiplexed by the demultiplexing sectionand travels through the first and second optical waveguides,, respectively, and then multiplexed at the multiplexing section, the multiplexed light is output from the output optical waveguideas modulated light So.

7 10 8 10 7 10 8 10 7 a b a b The first electrodecovers the first optical waveguidein a plan view, and the second electrodealso covers the second optical waveguidein a plan view. That is, the first electrodeis formed on the first optical waveguidevia a buffer layer (to be described later), and the second electrodeis also formed on the second optical waveguidevia a buffer layer. The first electrodeis connected to, for example, an AC signal, and can be referred to as a signal electrode. The second electrode is grounded, for example, and may be referred to as a “ground” electrode.

7 10 10 10 10 10 10 10 a b a b a b o. The electric signal (modulated signal) is input to the first electrode. The first and second optical waveguidesandare made of a material, such as lithium niobate having electro-optical effect, so that the refractive indices of the first and second optical waveguidesandare changed with +Δn and −Δn by an electric field applied to the first and second optical waveguidesand, with the result that a phase difference between the pair of optical waveguides changes. A signal light modulated by the change in the phase difference is output from the output optical waveguide

2 FIG. 1 b FIG.() 100 is a schematic cross-sectional view of the optical modulatortaken along line A-A′ of.

2 FIG. 100 1 2 3 4 1 2 1 2 10 10 2 10 10 a b r a b As illustrated in, the optical modulatorof the present embodiment has a multilayer structure including a substrate, a waveguide layer, a buffer layer, and an electrode layerwhich are laminated in this order. The substrateis, e.g., a sapphire substrate, and a waveguide layermade of a lithium niobate film is formed on the surface of the substrate. The waveguide layerhas the first and second optical waveguides,composed of a ridge portion. The width of the first and second optical waveguides,may be, e.g., 1 μm.

3 2 2 10 10 7 8 3 3 2 3 10 10 10 10 3 10 10 3 2 2 2 3 r a b a b a b a b r r 2 3 2 3 3 2 2 3 The buffer layeris formed on at least the upper surface of the ridge portionof the waveguide layerso as to prevent light propagating through the first and second optical waveguides,from being absorbed by the first electrodeor the second electrode. Therefore, the buffer layeronly needs to function as an intermediate layer between the optical waveguide and the signal electrode, and the material of the buffer layer can be widely selected as long as it is a non-metal. For example, the buffer layer may use a ceramic layer made of insulating materials such as metal oxides, metal nitrides, and metal carbides. The material of the buffer layer may be a crystalline material or an amorphous material. The buffer layeris preferably formed of a material having a lower refractive index than the waveguide layer, such as AlO, SiO, LaAlO, LaYO, ZnO, HfO, MgO, YO, and the like. The thickness of the buffer layer formed on the optical waveguide may be about 0.2 μm to 1.2 μm. In the present embodiment, the buffer layernot only covers the upper surfaces of the first and second optical waveguides,, but is also buried between the first and second optical waveguides,. That is, the buffer layeris also formed in a region that does not overlap with the first and second optical waveguidesandin a plan view. The buffer layercovers the entire region of the upper surface of the waveguide layerwhere the ridge portionis not formed, and the side surface of the ridge portionis also covered by the buffer layer.

4 7 8 7 2 10 10 10 3 8 2 10 10 10 3 r a a a r b b b The electrode layeris provided with the first electrodeand second electrode. The first electrodeis provided overlapping the ridge portioncorresponding to the first optical waveguideso as to modulate light traveling inside the first optical waveguideand opposed to the first optical waveguidethrough the buffer layer. The second electrodeis provided overlapping the ridge portioncorresponding to the second optical waveguideso as to modulate light traveling inside the second optical waveguideand opposed to the second optical waveguidethrough the buffer layer.

2 2 3 Although the waveguide layeris not particularly limited as long as it is an electro-optical material, it is preferably made of lithium niobite (LiNbO). This is because lithium niobate has a large electro-optical constant and is thus suitable as the constituent material of an optical device such as an optical modulator. Hereinafter, the configuration of the present invention when the waveguide layeris formed using a lithium niobate film will be described in detail.

1 1 Although the substrateis not particularly limited in type as long as it has a lower refractive index than the lithium niobate film, it is preferably a substrate on which the lithium niobate film can be formed as an epitaxial film. Specifically, the substrateis preferably a sapphire single crystal substrate or a silicon single-crystal substrate. The crystal orientation of the single-crystal substrate is not particularly limited. The lithium niobate film can be easily formed as a c-axis oriented epitaxial film on single-crystal substrates having different crystal orientations. Since the c-axis oriented lithium niobate film has three-fold symmetry, the underlying single-crystal substrate preferably has the same symmetry. Thus, the single-crystal sapphire substrate preferably has a c-plane, and the single-crystal silicon substrate preferably has a (111) surface.

The term “epitaxial film”, as used herein, refers to a film whose crystal orientation is aligned with respect to the underlying substrate or film. When the film plane is set to the X-Y plane and the film thickness direction is set to the Z-axis, the crystals are aligned in the X-axis, Y-axis, and Z-axis directions. For example, the existence of an epitaxial film can be confirmed by first measuring the peak intensity at the orientation position by 2θ-θX-ray diffraction and then confirming poles.

Specifically, first, in the 2θ-θX-ray diffraction measurement, all the peak intensities except for the target plane must be 10% or less, preferably 5% or less, of the maximum peak intensity on the target plane. For example, in a c-axis oriented epitaxial lithium niobate film, the peak intensities except for a (00L) plane are 10% or less, preferably 5% or less, of the maximum peak intensity on the (00L) plane. (00L) is a general term for (001), (002), and other equivalent planes.

3 3 Secondly, poles must be observed in the measurement. Under the condition where the peak intensities are measured at the first orientation position, only the orientation in a single direction is proved. Even if the first condition is satisfied, in the case of nonuniformity in the in-plane crystalline orientation, the X-ray intensity is not increased at a particular angle, and poles cannot be observed. Since LiNbOhas a trigonal crystal system, single-crystal LiNbO(014) has 3 poles. For the lithium niobate film, it is known that crystals rotated by 180° about the c-axis are epitaxially grown in a symmetrically-coupled twin crystal state. In this case, three poles are sym-metrically-coupled to form six poles. When the lithium niobate film is formed on a single-crystal silicon substrate having a (100) plane, the substrate has four-fold symmetry, and 4×3=12 poles are observed. In the present invention, the lithium niobate film epitaxially grown in the twin crystal state is also considered to be an epitaxial film.

The lithium niobate film has a composition of LixNbAyOz. A denotes an element other than Li, Nb, and O. The number x ranges from 0.5 to 1.2, preferably 0.9 to 1.05. The number y ranges from 0 to 0.5. The number z ranges from 1.5 to 4, preferably 2.5 to 3.5. Examples of the element A include K, Na, Rb, Cs, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Cr, Mo, W, Fe, Co, Ni, Zn, Sc, and Ce, alone or in combination.

10 10 a b The lithium niobate film preferably has a thickness of equal to or smaller than 2 μm. This is because a high-quality lithium niobate film having a thickness larger than 2 μm is difficult to form. The lithium niobate film having an excessively small thickness cannot completely confine light, allowing light to leak to the substrate or the buffer layer and thus to be guided therethrough. Application of an electric field to the lithium niobate film may therefore cause a small change in the effective refractive index of the optical waveguides (and). Thus, the lithium niobate film preferably has a thickness that is at least approximately one-tenth of the wavelength of light to be used.

2 3 It is desirable to form the lithium niobate film by a film forming method such as sputtering, CVD, or sol-gel process. If the c-axis of the lithium niobate film is oriented perpendicular to the main surface of the single-crystal substrate, an electric field is applied parallel to the c-axis, thereby changing the optical refractive index in proportion to the intensity of the electric field. If the single-crystal substrate is sapphire, the lithium niobate film is formed by epitaxial growth directly on the sapphire single-crystal substrate. If the single-crystal substrate is silicon, the lithium niobate film is formed by epitaxial growth on a cladding layer (not shown) formed on the substrate. The cladding layer (not shown) is made of material which has a lower refractive index than the lithium niobate film and should be suitable for epitaxial growth. For example, if the cladding layer (not shown) is made of YO, a lithium niobate film of high quality can be formed.

As a formation method for the lithium niobate film, there is known a method of thinly polishing or slicing the lithium niobate single crystal substrate. This method is advantageous in that the same characteristics as the single crystal can be obtained and can be applied to the present invention.

3 10 10 3 10 10 7 8 10 10 3 10 10 7 8 10 10 7 8 10 10 10 10 3 10 10 a b a b a b a b a b a b a b a b In this embodiment, the buffer layerhas recesses above the first and second optical waveguidesand. Specifically, the buffer layerprovided between the first and second optical waveguides,and the first and second electrodes,has a shape protruding downward at the first and second optical waveguides,. That is, the buffer layerprovided between the first and second optical waveguides,and the first and second electrodes,has a shape bent downward at the first and second optical waveguides,. The electrodes,on the optical waveguides,can be closer to the optical waveguides,through such a buffer layer, and can effectively apply an electric field to the optical waveguide, thereby locking the light in the optical waveguide,and suppressing the propagation loss of the light.

2 FIG. 3 10 10 3 3 a b As illustrated in, the amount of recess R of the buffer layeron the first and second optical waveguides,is 500 Angstrom or more, preferably 1000 Angstrom to 10000 Angstrom, and more preferably 1000 Angstrom to 2000 Angstrom. Here, the amount of recess R is the distance from the uppermost end of the upper surface of the buffer layerto the lowest end (lowest end of the depression) of the upper surface. In order to verify the relationship between the amount of recess R of the buffer layerand the propagation loss of light, the inventor of the present invention conducted the following experiment. Specifically, samples 1 to 3 and comparative example are electro-optical devices with the same structure except for the difference in the amount of depression of the buffer layer on the ridge-shaped optical waveguide.

amount of recess of the buffer layer on Light propagation the ridge-shaped optical waveguide loss Sample 1 1000 Angstrom 12 dB Sample 2 2000 Angstrom 12 dB Sample 3  500 Angstrom 14 dB comparative   0 Angstrom Poor light guide example

It can be seen from the table that when the amount of recess R is more than 1000 Angstrom, especially when 1000 Angstrom-2000 Angstrom, the light propagation loss is lower. Regarding the reduction of the propagation loss, although the cause has not been fully clarified, it can be presumed that the recess can make the electric field more effectively applied to the optical waveguide. In addition, the recess can prevent the light leaking upward from the optical waveguide from being reflected and then returning and interacting with each other. Therefore, it is possible to prevent the light leaking from the signal light from being scattered and reflected to cause adverse effects on the signal light. When the amount of recess R is 0 Angstrom, that is, no recess is formed, the propagation loss of light is relatively large. This may be due to the fact that the light leaking upward from the optical waveguide is reflected and then returned to interact with each other, and the light leaking from the signal light is scattered and reflected, which adversely affects the signal light. In addition, from the viewpoint of ease of manufacture, the amount of recess R is preferably 10000 Angstrom or less.

1 3 10 10 1 3 10 10 10 10 7 8 10 10 10 10 10 10 a b a b a b a b a b a b In addition, in the present embodiment, the distance from the surface (upper surface) of the substrateto the uppermost portion of the buffer layeron the optical waveguides,is smaller than the distance from the surface (upper surface) of the substrateto the uppermost portion of the buffer layeron the portion where the optical waveguide is not formed (i.e., between the optical waveguides,and the part other than the optical waveguides,). With this arrangement, the electrodes,on the optical waveguides,can also be closer to the optical waveguides,, and an electric field can be effectively applied to the optical waveguides, thereby locking the light in the optical waveguide,and suppressing the propagation loss of the light.

3 FIG. 4 FIG. 3 FIG. 4 FIG. 200 200 200 310 310 1 34 307 310 309 309 34 307 310 310 200 31 32 33 34 31 32 31 32 10 10 2 a b a a b a a a b r. is a plan view of an optical switching deviceaccording to another embodiment of the present invention.is a schematic cross-sectional view of the optical switching devicetaken along line B-B′ of. The optical switching deviceincludes: first and second optical waveguides,formed on the substrate, a film heater (film electrode)provided along the first optical waveguide, and wirings,for energizing the film heater. The film heater (the film electrode)is provided directly above the first optical waveguideso as to overlap the first optical waveguidein a plan view. As illustrated in, the optical switching deviceaccording to the present embodiment has a multilayer structure including a substrate, a waveguide layer, a buffer layer, and a film electrode layerwhich are laminated in this order. The substrateis, e.g., a silicon substrate, and the waveguide layermade of a lithium niobate film is formed on the surface of the substrate. The waveguide layerhas the first and second optical waveguidesandeach formed by a ridge portion

200 100 4 200 34 307 307 310 310 310 310 307 a b a a The optical switching deviceuses the same ridge-shaped lithium niobate film as the optical modulatorof the first embodiment, and has a structure in which the optical waveguide is embedded in the buffer layer. There is no electrode layerin the optical switching device, and a film heater (film electrode)is used instead. That is, a film heateris formed on the upper layer of a part of the optical waveguidesand(the optical waveguidein this embodiment), and the optical waveguidecan be heated by flowing a current through the film heater.

200 307 310 310 200 33 307 200 a b In the optical switching device, in the state where the film heateris turned on, the phase of the light passing through the optical waveguideis shifted, and the light is combined with another optical waveguideto switch the light. In the optical switching device, by making the buffer layerhave recesses on the side of the film heater, the propagation loss of light can also be reduced, thereby obtaining the optical switching devicewith excellent performance.

In addition, as the material used for the film heater, a heat-sensitive material such as MnNiCo-based oxide can be used, or a platinum heater can be used.

100 3 10 10 10 10 3 3 3 3 10 10 3 10 10 3 3 3 10 10 6 FIG. a b a b a b b a b a a b b a b a b Although the present invention has been specifically described above in conjunction with the drawings and embodiments, it can be understood that the above description does not limit the present invention in any form. For example, in the above description of the optical modulator, the first electrode is used as a signal electrode and the second electrode is used as a ground electrode. However, it is not limited to this, and the first and second electrodes may be any electrodes that apply an electric field to the optical waveguide. Further, as shown in, the recesses of the buffer layerabove the adjacent first and second optical waveguidesandcan form as a whole. That is, one recess across the optical waveguidesandcan be formed. In addition, buffer layermay consist of buffer layerand buffer layer, wherein the buffer layeris formed in the same layer with the optical waveguidesand, and the buffer layeris formed on optical waveguidesandand the buffer layer. Both buffer layerand the buffer layermay have a recess across the optical waveguidesand. Those skilled in the art can make modifications and changes to the present invention as needed without departing from the essential spirit and scope of the present invention, and these modifications and changes fall within the scope of the present invention.

100 optical modulator 1 substrate 2 waveguide layer 2 r ridge portion 3 buffer layer 4 electrode layer 7 first electrode 8 second electrode 10 Mach-Zehnder optical waveguide 10 a first optical waveguide 10 b second optical waveguide 10 c demultiplexing section 10 d multiplexing section 10 i input optical waveguide 10 o output optical waveguide 200 optical switching device 31 substrate 32 waveguide layer 32 r ridge portion 33 buffer layer 34 film electrode layer 307 film heater 309 309 a b ,wirings 310 310 a b ,first and second optical waveguides